1. Field of the Invention
This invention relates to a radiation image read-out apparatus and a radiation image convertor panel, and more particularly to a radiation image read-out apparatus in which stimulated emission emitted from a radiation image convertor panel upon exposure to stimulating light is detected by a photodetector and a radiation image convertor panel to be used in the radiation image read-out apparatus.
2. Description of the Related Art
When certain kinds of phosphors are exposed to radiation such as X-rays, they store a part of energy of the radiation. Then when the phosphors which have been exposed to the radiation are exposed to stimulating light such as visible light, light having a wavelength shorter than the stimulating light is emitted from the phosphors in proportion to the stored energy of the radiation. Phosphors exhibiting such properties are generally referred to as “stimulable phosphors”. In this specification, the light emitted from the stimulable phosphors upon stimulation thereof will be referred to as “stimulated emission”. There has been known as a CR (computed radiography) a radiation image recording and reproducing system, comprising a radiation image recording apparatus and a radiation image read-out apparatus, in which a layer of the stimulable phosphors is exposed to a radiation passing through an object such as a human body to have a radiation image of the object once stored on the stimulable phosphor layer as a latent image, stimulating light such as a laser beam is subsequently projected onto the stimulable phosphor layer, and the stimulated emission emitted from the stimulable phosphor layer is photoelectrically detected, thereby obtaining an image signal (a radiation image signal) representing a radiation image of the object. There has been known a radiation image convertor panel comprising a stimulable phosphor layer formed on a substrate as a recording medium employed in the radiation image recording and reproducing system.
There has been known a radiation image read-out apparatus in which red stimulating light emitted from a semiconductor laser is projected onto a radiation image convertor panel and stimulated emission emitted from the radiation image convertor panel upon exposure to the red stimulating light is detected by a photomultiplier through a stimulating light cut filter which transmits the stimulated emission and cuts the red stimulating light, whereby a radiation image signal representing a radiation image recorded on the radiation image convertor panel is obtained.
Though the red stimulating light emitted from the semiconductor laser is cut by the stimulating light cut filter, components of the light emitted from the semiconductor laser longer than the red stimulating light in wavelength can pass through the stimulating light cut filter to impinge upon the photomultiplier. However since being very low in sensitivity to light longer in wavelength than the red region, the photomultiplier never detects light components longer in wavelength than the red region when detecting stimulated emission which is shorter than the stimulating light in wavelength.
In an attempt to meet a demand for miniaturization of the radiation image read-out apparatus, there has been studied an apparatus in which stimulated emission emitted from a radiation image convertor panel is imaged on a line sensor formed by a number of CCD elements and an image signal representing a radiation image recorded on the radiation image convertor panel is obtained through the line sensor.
Since being formed by a diode having a sensitivity to light longer in wavelength than the stimulating light, the CCD element has a sensitivity to not only a visible region but also light longer in wavelength than the red region and detects light components longer than the red stimulating light in wavelength passing through the stimulating light cut filter to impinge upon the line sensor, whereby signal components representing the longer side light components can mix in the image signal as a noise component. Further, these inventor's analysis has revealed that the longer side light components which can generate a noise component in the image signal include the following components.    (1) Those generated from the stimulating light source together with the stimulating light.    (2) Those generated from a radiation image convertor panel exposed to the stimulating light and believed to be mainly of fluorescence.    (3) Those generated from a condenser lens exposed to the stimulating light and believed to be mainly of fluorescence.    (4) Those generated from foreign matter adhering to the radiation image convertor panel and/or the condenser lens exposed to the stimulating light and believed to be mainly of fluorescence.
Those of (2) and (3) are equivalent to the intensity of stimulated emission emitted from a radiation image convertor panel exposed to radiation of several mr, and those of (4) are equivalent to the intensity of stimulated emission emitted from a radiation image convertor panel exposed to radiation of several tens mr. Accordingly, it is required to attenuate the intensity of the light components longer in wavelength than the stimulating light to preferably not higher than 1/100 and more preferably not higher than 1/1000. Further, it has been found that the main part of the light components longer in wavelength than the stimulating light is in the range not shorter than 800 nm and not longer 900 nm in wavelength.
This problem is involved not only when the stimulated emission is detected by the CCD elements but also when the stimulated emission is detected by other photodetectors which have a sensitivity to light longer in wavelength than the stimulating light.